Hitherto, electrically insulating tapes obtained by applying a tack adhesive to a surface of a plastic film and cutting such into a suitable width have been used as electrically insulating materials for coating electrical wieres or splicing cables, etc.
Recently, improvement of various types of properties including the heat resistance of the electrically insulating materials has been further required. However, this requirement cannot often be met with the above-described electrically insulating tapes.
Hence, in order to meet this requirement, electrically insulating materials comprising a polyimide film to which a silicone resin adhesive is applied or a thin layer of tetrafluoroethylene-hexafluoropropylene copolymer (referred to as FEP, hereinafter) is provided have been used. These electrically insulating materials comprising a polyimide film as a main component have improved heat resistance as compared with the above-described electrically insulating tapes. however, since the silicone resin adhesive or FEP has inferior heat resistance to that of the polyimide resin, use of such materials is restricted to use at a temperature less than their heat resistant temperature. Accordingly, they are not satisfactory, yet, because the excellent heat resistance of the polyimide resin itself cannot be utilized.